1. Field of the Invention
This invention relates to a method and a circuit for processing pulses by applying the technique of weighted acquisition for forming an image. The pulses are particularly caused by a radioactive source and produced by an imaging radiation detector, such as a scintillation camera for detecting gamma rays.
2. Description of the Prior Art
Radiation detectors are widely used as diagnostic tools for analyzing the distribution of a radiation-emitting substance in an object under study, such as for the nuclear medical diagnosis of a human body organ. A typical radiation detector of a type to which the present invention relates is a commercial version of the Anger-type scintillation camera, the basic principles of which are described in Anger U.S. Pat. No. 3,011,057.
Such a scintillation camera can take a "picture" of the distribution of radioactivity throughout an object under investigation, such as an organ of the human body which has taken up a diagnostic quantity of a radioactive isotope. As individual gamma rays are emitted from the distributed radioactivity in the object and pass through a collimator, they produce scintillation events in a thin planar scintillation crystal. The events are detected by photodetectors positioned behind the crystal.
Electronic circuitry translates the outputs of the photodetectors into X and Y coordinate signals which indicate the position in the crystal of each event and a Z signal which indicates generally the energy of the event and is used to determine whether the event falls within a preselected energy range (window). A picture of the radioactivity distribution in the object may be obtained by coupling the X and Y signals which fall within the preselected energy window to a display, such as a cathode ray oscilloscope which displays the individual scintillation events as spots positioned in accordance with the coordinate signals. The detection circuitry typically provides for integrating a large number of spots onto photographic film.
In modern scintillation cameras which comprise circuitry for energy and linear spatial distortion correction, such as described in Stoub et al. U.S. Pat. No. 4,298,944, Del Medico et al. U.S. Pat. No. 4,316,257 or Arseneau U.S. Pat. No. 4,323,977 for example, the Z signal is tested against a set of three energy signal windows (single channel analyzer) which are pre-set according to the isotope being imaged. Thus events having Z signals within any window are included as counts in the image, while all other events are excluded. The ultimate purpose of this test is two-fold: (1) to include primary, (photopeak) events in the image and (2) to exclude scattered gamma ray and fluorescent X-ray events from the image. The capability of scintillation cameras for making this window test has been decisive for nuclear medical imaging. Image intensifier cameras with outstanding spatial resolution, but without any capability for this test, have all been notable failures in nuclear radiography.
However, even scintillation cameras which comprise circuitry for linear energy and spatial distortion correction, are not able to perfectly accept all photopeak events and reject all others. This fact is traceable to the very nature of scattering and to the finite energy resolution of the camera scintillator. Normally each scintillation camera has only a finite resolution, which blurs both photopeak and scatter. As the amount of scatter material increases, or as the depth of the gamma source increases, the relative proportion of scatter events in the image also increases. The inclusion of scatter events in addition to non-scatter events in an image degrades lesion visibility, image contrast and system resolution.
One method, as a former attempt of reducing the scatter photon contribution to the events measured under the photopeak, is to raise the baseline of the Pulse Height Analyzer. But this necessarily results in a lower sensitivity because some primary photons are excluded when the baseline raises.
Another method, as an attempt of reducing the scatter influence, is the so called "window shifting". This means that for each used isotope a specific window is determined which is supposed to be the "best one" with regard to photopeak-to-scatter ratio. But this method, which for example is described in a study entitled "Optimizing the Window of an Anger Camera for .sup.99m Tc" by Theodore P. Sander et al., Journal of Nuclear Medicine, Vol. 12, No. 11, November 1971, 703-706, or in a study entitled "Effect of Pulse-Height Selection on Lesion Detection Performance" by F. D. Roll o et al., Journal of Nuclear Medicine, Vol. 12, No. 10, October 1971, 690-696, is also not satisfying.
A further method as an attempt of reducing the scatter influence is the so called "method of scatter subtraction". To compensate for the magnitude of scatter two windows are set, a first window for the photopeak together with scatter and the second window for scatter alone. The answer of the second window is then subtracted from the answer of the first window. But this scatter subtraction method which is for example described in a study entitled "Effects of Scatter Subtraction on Image Contrast" by Francis B. Atkins et al., Journal of Nuclear Medicine, Vol. 16, No. 1, January 1975, 102-104, or in a study entitled "Reduction of the Effects of Scattered Radiation on a Sodium Iodide Imaging System" by Peter Bloch et al., Journal of Nuclear Medicine, Vol. 14, No. 2, February 1973, 67-72, is also not satisfying.
A more successful method of reducing the scatter influence could have been the "method of weighted acquisition", which method is for example described in a paper entitled "Aspects of Imaging and Counting in Nuclear Medicine Using Scintillation and Semiconductor Detectors", by R. N. Beck et al., which paper was presented at the 13th Scintillation and Semiconductor Counter Symposium, Mar. 1-3, 1972, in Washington, DC. Another study discussing the "method" of weighted acquisition" is "Advances in Fundamental Aspects of Imaging Systems and Techniques", by R. N. Beck et al., reprint IAEA-SM-164/301 from "Medical Radioisotopes Scintigraphy 1972" Vol. 1, pages 3-45, especially pages 29, 30, 44 and 45.
But, the "method of weighted acquisition" was never brought to practical implementation. This may have been due to the fact, that this method was not well done in the prior art such that a reduction to practice was impossible till today.